Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity

A family of racemic and enantiopure indium complexes <b>1</b>–<b>11</b> bearing bulky chiral diaminoaryloxy ligands, H­(NNO_R), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO_R)­InX]₂[μ-Y]­[μ-OEt] (<b>5</b>, R = ^<i>t</i>Bu, X = Y = Cl; <b>8</b>, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly­(lactic acid) (PLA) at room temperature. In particular, complex <b>5</b> showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex <b>8</b>. Importantly, the selectivities obtained for the ROP of racemic LA with (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> and (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>8</b> are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes

Mechanism of Living Lactide Polymerization by Dinuclear Indium Catalysts and Its Impact on Isoselectivity

A family of racemic and enantiopure indium complexes <b>1</b>–<b>11</b> bearing bulky chiral diaminoaryloxy ligands, H­(NNO_R), were synthesized and fully characterized. Investigation of both the mono- and the bis-alkoxy-bridged complexes [(NNO_R)­InX]₂[μ-Y]­[μ-OEt] (<b>5</b>, R = ^<i>t</i>Bu, X = Y = Cl; <b>8</b>, R = Me, X = I, Y = OEt) by variable temperature, 2D NOESY, and PGSE NMR spectroscopy confirmed dinuclear structures in solution analogous to those obtained by single-crystal X-ray crystallography. The dinuclear complexes in the family were highly active catalysts for the ring-opening polymerization (ROP) of lactide (LA) to form poly­(lactic acid) (PLA) at room temperature. In particular, complex <b>5</b> showed living polymerization behavior over a large molecular weight range. A detailed investigation of catalyst stereoselectivity showed that, although (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> is highly selective for l-LA, only atactic PLA is obtained in the polymerization of racemic LA. No such selectivity was observed for complex <b>8</b>. Importantly, the selectivities obtained for the ROP of racemic LA with (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>5</b> and (<i>R</i>,<i>R</i>/<i>R</i>,<i>R</i>)-<b>8</b> are different and, along with kinetics investigations, suggest a dinuclear propagating species for these complexes

Theoretical Investigation of Lactide Ring-Opening Polymerization Induced by a Dinuclear Indium Catalyst

A DFT study of the ring-opening polymerization of lactide (LA) induced by a dinuclear indium catalyst supported by a chiral diamino phenoxy ligand, [(NN_HO)­InCl]₂(μ-Cl)­(μ-OEt) (<b>1</b>), is reported. The nature of the active catalyst, mononuclear vs dinuclear, was investigated and was shown to be dinuclear because of the high energetic cost of its dissociation. The selectivity of the system was investigated for the polymerization of LA with the dinuclear (<i>R,R</i>/<i>R,R</i>)-<b>1</b> catalyst. In complete agreement with experimental results we observed that (1) selectivity is controlled by the nucleophilic addition of LA to the alcoholate, resulting in the chain-end control of polymerization, (2) a slight kinetic preference for the polymerization of l-LA over d-LA is found that translates to a <i>k</i>_rel value of ∼14, which is identical with the experimental value, and (3) when <i>rac</i>-LA is used, no clear preference for d- vs l-LA insertion is found, leading to isotactic PLA

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (<b>1</b>), along with its zinc analogue (<b>2</b>), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis­(diamino)­phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (<b>7</b>) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (<i>rac</i>-LA) to afford highly heterotactic poly­(lactic acid) (PLA; <i>P</i>_r > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from <i>meso</i>-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed

Redox Control of Group 4 Metal Ring-Opening Polymerization Activity toward l‑Lactide and ε‑Caprolactone

The activity of several group 4 metal alkoxide complexes supported by ferrocene-based ligands was controlled using redox reagents during the ring-opening polymerization of l-lactide and ε-caprolactone. Switching in situ between the oxidized and reduced forms of a metal complex resulted in a change in the corresponding rate of polymerization. Opposite behavior was observed for each monomer used. One-pot copolymerization of the two monomers to give block copolymers was also achieved